ENG 331 Group 7’s Final Project

Technology Overview

Since the cell phone was created the technology inside one has been ever changing, even today.  Believe it or not, a cell phone is actually a very complex radio.  In this section of our site we will discuss some of the technologies in and around a cell phone that allows us to communicate with almost everyone, anywhere at any time on the globe while also providing an array of functions to the user.  Some sophisticated technologies are needed to provide us with the cutting edge communication system we tend to take for granted.


Half/Full Duplex

Half/Full Duplex systems are differentiated by how they use frequencies for communication between electronic devices.  Walkie talkies and CB radios are half duplex devices; this translates to only one frequency being shared by two users, (Brain, Tyson, and Layton; How Cell Phones Work), creating a limit in the usability, only one user can talk at a time.  A full duplex system was created for cell phones to allow users to talk at the same time.  An example of a technique used to accomplish this ability is FDMA (Frequency Division Multiple Access), shown in the figure below.

FDMA

FDMA dedicates a different frequency to the two transmitters, the one in the cell phone and the one at the tower, used to communicate at one end of the link.  In other words, try to image the transmission and receiving of data at the cell phone as traffic flow and each frequency is a one way road.  The utilization of two frequencies allows two paths for data to flow, from the cell phone to the currently connected tower.  These two separate paths enable data to leave and enter the cell phone antenna simultaneously.  With this technology applied at each end of the communication link, CellA <-> TowerA & TowerB <-> CellB, each user is able to talk at the same time making the communication experience as real as if each user was having an in person conversation.


Range

Every wonder how in some places you can’t find a signal and in others you have a very strong signal?  This is a direct result to placement of towers and surrounding terrain.  The effects of electromagnetic wave propagation and interference/reflection/diffraction differentials make the math for getting our elaborate communications systems to work in a cohesive manner extremely complicated.  For this overview I will not go into the computations but focus more on the higher level restrictions of range in the cell phone’s wireless communication domain.

The physics of a propagating wave is of some importance to placement of towers.  It is known that as a wave travels away from its source it expands in a circular form while diminishing in magnitude, shown in figures below.

Wave Propagation & Power Decrease

*** Note: l, 2l, 3l, 4l, 5l correspond in the two plots above. ***

These properties are commonly analyzed with dropping a stone in a pond, such that from the location of the stone, location of tower, a circular wave propagates away while getting weaker the further it travels from the stone.  Applying these physical facts we can conclude that there is a limit of distance from a tower the cell phone can be before the signal strength decreases to a point where it can’t communicate with the tower.  The area that a tower can accommodate is conveniently known as cells (Poole, 2006).  Representing a portion of coverage, cells are formed in this hexagonal way:

Hex Grid

The edges of the cells in the figure above are not represented exactly, but are to the point of conceptual understanding.  By utilizing this hexagonal format and the low power/low range transmitters at the towers, non adjacent cells can reuse the same frequencies (Brain, Tyson, and Layton; How Cell Phones Work).  Depending on the power of the transmitter on the tower and the terrain around it, cell areas vary greatly.  In a sparse populated area, fewer cells are used and are not orientated in this hexagonal fashion but in such a way as to provide service to the majority of the population.  So, from this information we can see that in a rural area there would be a possibility of you being outside the area of a cell and therefore have no signal and be unable to make a call.

Apart from the positioning of towers the surrounding terrain plays a part in available connectivity.  Again, wave propagation physics plays a part in evaluating what happens to your signal strength with surrounding obstacles.  Shown in the figure below is an instance of diffraction due to a peak or ridge:

Diffraction

The area shown as “Shadow area” is where you could have problems connecting to the nearest tower.  A peak/ridge greatly reduces the signal strength to the area below the line of sight from the cell phone to the tower.  Advancements in technology are allowing us to decrease the effect of this situation but it is still a concern in the wireless domain.


1G/2G/3G/4G

1G (First Generation), 2G (Second Generation), 3G (Third Generation), and 4G (Forth Generation) are different network protocols used by cell phones.  Each later generation builds on the previous to accommodate the ever growing demand for quality of service and faster transfer rates in the market.  While 4G is not yet out officially it is well into the development stages.

1G was the analog telecommunication standard, introduced in the 1980s, until later being replaced by 2G digital telecommunications.  The 1G network communicated from cell phone to tower by analog signals using the Advanced Mobile Phone System (AMPS).  In this system, FDMA was used to obtain the full duplex connection needed, to achieve two way talk ability, and allow use for multiple users in the same cell at the same time.  With this technology in place and usable the idea of cell phones caught on in the public and the demand skyrocketed, as did the want for more functionality.  Seeing how this 1G network protocol just provided voice only technology a change had to be made.

2G telecommunication networks were commercially launched in the early 1990s, bringing a lot of new advantages.  There were three main changes made to the preexisting network; communication from cell phone to tower were digital signals, more efficient use of bandwidth, and the introduction of data services, text messaging.  With the big increase of users this new technology also came to improve access methods.  In addition to FDMA came TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access).  TDMA, shown in the figure below, assigns each call one requency and alternates between a transmitting and receiving channel at a periodic rate.

TDMA

While this method achieves a full duplex connection and doubles maximum possible simultaneous users, it limits the throughput of data and can reduce signal quality.  CDMA, the most complex of the three protocols, gives a unique code to each call and “spreads” the call over all available frequencies (Brain, Tyson, and Layton; How Cell Phones Work).  This implementation of code allows different calls to use the same frequency better utilizing the finite frequency spectrum.  As demand for a true multimedia and web based application accessible cell phone, smartphone, grew it was meet by 3G.  By making modifications to the existing protocols, this new technology was able to obtain the appropriate transfer rate needed to support video calls and broadband wireless data transfer.  Updates to the CDMA network protocol produced CDMA2000 (based on 2G CDMA), WCDMA (Wideband Code Division Multiple Access) and a TDMA CDMA hybrid TD SCDMA (Time Division Synchronous Code Division Multiple Access).  These protocols are the network we now know today with 3G being the standard in telecommunications.  With all the abilities 3G provides who knows what the next generation of network will bring.

4G is going to be the next complete evolution in wireless communication.  It will be the first complete replacement of current networks since FDMA first came out in the 1980s.  With this overhaul providing IP oriented voice, data and multimedia steaming at an incredibly high transfer rate who knows what even the next steeping stone will bring while getting us closer to a more efficient lifestyle.

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